18 July 1998
Nelson Mandela first married in 1944. He and his wife Evelyn had 2 children and divorced in 1957. A year later, he married Winnie Madikizela-Mandela. Their marriage survived his 27 years in prison, but ended in divorce in 1996. Shortly after his release from prison, Mandela met Graça Machel, the widow of former Mozambican president, Samora Machel. Apart from her involvement in the Mozambican liberation struggle, Graca Machel is known as an international advocate for women and children rights, and headed the first United Nations study on the impact of war on children. The couple decided to tie the knot on Nelson Mandela's 80th birthday. Their marriage was announced at his birthday celebration, which was attended by 2,000 international guests.
References

Graighall Wallis, F. (2000) Nuusdagboek: feite en fratse oor 1000 jaar,| 

BBC News, (1998), Africa Mandela weds on his 80th birthday, 19 July [online], Available at www.bbc.co.uk [Accessed: 10 July 2013]